Polyamides, useful as molding resins and as film and fiber formers, are made commercially by reacting a diacid or diacid precursor with a diamine. In one process a diacid is initially reacted with a diamine in an aqueous media to form a solid polyammonium salt which precipitates from the reaction mixture. The solid is then heated successively in an autoclave reactor under pressure to control explosive initial dehydration and subsequently under reduced pressure to effect conversion of the polyammonium salt to a polyamide. This requires expensive equipment and very careful process control in order to optimize heat and mass transfer. The high melting solid salt which initially can't be stirred undergoes almost explosive dehydration as the reactor temperature is increased. The solid salt is converted to a mobile liquid which increases in viscosity as additional water is removed as high pressure steam. An alternate approach to the use of autoclaves consists of dispersing the polyammonium salts in highly polar solvents and effecting dehydration and solubilization by elevating the temperature of the solution. This process eliminates the need for autoclaves but invokes the use of very special high boiling solvents. The product must ultimately be recovered from solution as a solid. The expensive, high boiling, polar and frequently toxic solvents must be completely removed from the product and be very efficiently recycled to control manufacturing costs. In still another version, the diacids may be first converted to diacid chloride derivatives which can be reacted in solution or interfacially with diamines at relatively low temperatures to yield polyamides. Very corrosive hydrogen chloride is produced as a by-product of this reaction. The by-product acid must be completely removed from the solution or dispersion of polymer and the polymer must be separated from the reaction solvent. Again the solvent must be efficiently recycled. The process of the present invention on the other hand is smooth and rapid, and it is widely applicable, being limited, apparently, only by the ultimate nature of the final product, which must be melt processable, i.e., it must have a melting point below the thermal decomposition temperature. In practice, this excludes only a relatively few polyamides, namely, high melting materials such as polyamides containing units derived from p-phenylene diamine, or m-phenylene diamine and aromatic diacids, such as isophthalic and terephthalic acids, or polyamides derived from the 4,4'-dicarboxylic acid of diphenyl, and the like. In general, subject to the foregoing requirement that the ultimate product be melt processable, the dibasic acid diester should be a diaryl ester of a dicarboxylic acid containing 5 carbon atoms and above, i.e., glutaric, adipic, sebacic acid etc. The di-primary amines should include those of 4 carbon atoms and above, i.e., tetramethylene, pentamethylene, hexamethylene diamine, etc.
The novel products of the process are also features of the invention, many of them being transparent, tough, and solvent-resistant.